Heretofore, as a method for producing an ingot required to have a high cleanliness, known is an electroslag remelting process (hereinafter this may be referred to as ESR process). The ESR process is a process for producing a clean ingot by melting an electrode by the resistance heat of a molten slag and then successively solidifying the melt in a water-cooling mold. In the ESR process, a slag having a suitable specific resistance, melting point and viscosity must be used, and in general, a CaF2—CaO—Al2O3 ternary slag is used. However, the slag was developed for melting a Fe-base alloy or a Ni-base alloy and therefore has the property that its melting point is high.
On the other hand, a large-size ingot (1 ton or more) of a copper alloy is generally produced by melting according to a mold casting method or a continuous casting method. A copper alloy has a higher thermal conductivity than steel and may often have cast defects inside it as its solidification speed is high. A copper alloy produced by melting an inexpensive material such as scrap or the like often contains S in an amount of several tens ppm or more, and its intergranular strength lowers owing to S segregating in the grain boundary therein, and therefore its high temperature ductility greatly worsens. Further, certain copper alloys may have a large quantity of eutectic compounds formed therein during solidification, and when a large-size ingot of such a copper alloy is produced by melting according to a mold casting method, then the eutectic compounds may grow to coarse grains to worsen the workability of the alloy. In addition, the copper alloy of the type has a low strength at a high temperature, and therefore its ingot is difficult to draw in a continuous casting method.
Accordingly, investigations have been made regarding the applicability of the ESR process to a copper alloy; however, since a copper alloy has a low melting point, a slag having a high melting point as in the above could not be applied to melting by ESR for a copper alloy. In fact, there are known a few examples of melting by ESR for a copper alloy, and there are few reports relating to a slag for copper alloy; however, in some literatures such as Patent Literature 1 and Patent Literature 2, a slag for copper alloy is proposed.
In Patent Literature 1, proposed is an electroslag remelting method for copper and copper alloy, comprising simultaneously adding one or more of, by mass, SiO2: 5 to 40%, TiO2: at most 10%, Na2O: at most 5%, MnO: at most 5%, BaO: at most 5%, and MgO: at most 10%, to a basic composition of Na3AlF6—CaF2—Al2O3 or CaF2—LiF—Al2O3.
In Patent Literature 2, proposed is an electroslag casting method for copper or copper alloy, comprising using a slag having a compositional ratio, by mass, of SiO2: 30 to 40%, MnO: 9 to 15%, Al2O3: 1 to 8%, TiO2: ≦10%, BaO: ≦1%, CaO: 15 to 80%, CaF2: 3 to 7%, and MgO: ≦2%.    Patent Literature 1: JP-A-61-183419    Patent Literature 2: JP-A-53-48926